\documentclass{article}
\usepackage{amssymb,amsmath,amscd,concmath}
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\begin{document}
\begin{equation*}\textbf{K}_{k} = \hat{\textbf{P}}_{k|k-1}\textbf{H}^{T}\textbf{S}_{k}^{-1} \end{equation*}
\end{document}
